The present invention relates to a method for controlling certain machine parameters in friction welding as well as an apparatus for performing the method.
In friction welding the workpieces are rubbed against each other in an oscillating translational motion until the material melts in the contact zone and flows together in the contact area. Then the vibration is terminated, the process resulting in an integral connection during cooling down. Characteristic parameters for this method are the joining force F resulting in a joining pressure and the relative motion x of the workpieces in the rubbing direction. This is shown in FIG. 1. Both parameters are time-responsive. As a rule, the joining pressure is maintained constant. The relative motion of the workpieces, however, is substantially defined by a sinus oscillation having a predetermined amplitude and frequency.
The vibration welding procedure is performed in a plurality of steps which follow each other. In a first step the members to be joined are transported to receiving means of the machine and fixedly secured thereto. Then both members are displaced towards each other until they come into contact. After a short delay time under a static contact force the vibration is initiated. The material first softens by friction and then more and more due to a hysteretic sheering heating. During this process the molten material is urged to flow into the welding bead by the joining pressure exerted. Consequently the workpieces are displaced towards each other in z direction; the joining displacement s increases. After some seconds the vibration is terminated. The piece still remains under the joining pressure or, respectively, holding pressure, for a short time to allow for proper cooling down. Subsequently the receiving means are opened and the piece may be removed.
In order to obtain high strength connections the material must undergo a sufficient melting process and joining pressure. Hitherto these requirements have been observed by maintaining at least a minimum joining distance. Further, the formation of a uniform and well-shaped welding bead is visually inspected. However, this is successful with simple geometric designs of the workpieces only and with workpieces which possess very little tolerances. More complicate workpieces, however, often have internal welding zones. Furthermore, workpieces made by an injection molding process exhibit substantial tolerances in dimensions. Therefore an optical inspection of the bead along the outer face of the piece in combination with a minimum joining displacement distance is not sufficient anymore to rely on.